Individual molecules of the gases which constitute air can acquire an electrical charge and become positive or negative ions depending on whether a deficiency or an excess of electrons has been imparted to the molecule. Some ions of each type are usually present in outdoor air as a result of static electricity discharges and other natural causes. The degree of air ionization and the relative proportions of positive and negative ions vary widely under natural conditions in response to changes in atmospheric and environmental factors.
A relatively high level of air ionization has beneficial effects. Ions tend to remove particulate contaminants, such as smoke particles or pollens for example, from air by transferring charge to such particles. The charged particles are then electrically attracted to nearby surfaces that are electrically neutral or oppositely charged and are deposited against such surfaces. Air rich in negative ions in particular is also believed to have beneficial physiological effects.
Air inside buildings tends to become stale and unpleasant to breathe, the effect being attributable at least in part to depletion of the ion content. A variety of air ionizers have heretofore been developed to counteract this effect and also to purify air by removing particulate contaminants in the manner previously described. Typically, such air ionizers have an electrode coupled to a D.C. high voltage source and having one or more sharp points. The intense electrical field adjacent the point or points breaks down nearby gas molecules to form positive or negative ions depending on the polarity of the high voltage. Electrostatic repulsion from the similarly charged electrode and from each other, together with electrostatic attraction to neutral objects such as the walls of the room, then causes such ions to disperse outwardly from the ionizer. Air blowers are sometimes provided to accelerate such dispersion and to extend the effective range of the ionizer.
Ionizers designed specifically for the above described purposes usually produce only the physiologically beneficial negative ions. Another, increasingly important application of air ionizers requires that ions of both polarities be generated. In particular, air ionizers can also be extremely useful for suppressing build-up of electrostatic charges on objects in a room.
Objects, including humans, situated in a room very often acquire a sizable electrostatic charge which may have a magnitude of several thousand volts or more. Such charging of non-conductive objects may be caused by movement and the accompanying friction while induction and discharges from other objects may impart charge to ungrounded conductors. Sizable electrostatic charge accumulations of this kind can be highly undesirable for a number of reasons. Sudden discharges, although not usually harmful, are distinctly unpleasant to people. Electrostatic charge can also interfere with the operation of electrical devices ranging from phonographs to computers.
Problems caused by electrostatic charge build-up have become particularly acute in the manufacture of certain products such as in facilities, known as clean rooms, in which miniaturized semiconductor circuit components are fabricated. Static discharges frequently damage or destroy the minute conductive paths in microcircuits or the like and the electric field associated with electrostatic charge can have other adverse effects as well. Dust particles or other contaminants, for example, are attracted to a charged area of a semiconductor wafer or the like and then adhere to the area by electrostatic attraction. Suppression of electrostatic charge build-up is a practical necessity in clean rooms to avoid unacceptably high product losses. For this purpose, a variety of techniques are utilized in combination. Careful arrangements are made for the grounding of conductive or somewhat conductive objects in the room including personnel and antistatic structural materials and specialized clothing are usually utilized. Establishing and maintaining a high level of air ionization is also recognized to be a very effective technique for suppressing electrostatic charge.
An accumulation of electrostatic charge on an object can be neutralized by charge exchange with air ions of opposite polarity which are attracted by the electrical field associated with the electrostatic charge. A high level of both positive and negative ions is most effective for this purpose as the electrostatic charges to be neutralized may be of either polarity. Prior bipolar air ionizers and the methods under which such ionizers are operated are not as effective for this purpose, or for certain other purposes, as would be desirable.
Some prior procedures and apparatus that produce air ions of both polarities are intended to impart electrostatic charges to objects. Prior U.S. Pat. No. 3,936,698, for example, teaches the treatment of medical patients by cyclically directing negative ions from a first ionizing electrode to the patient for intervals of the order of five minutes during which the patient acquires a negative charge. Positive ions from a second electrode are then directed at the patient between such periods to remove the electrical charge. Such procedures are obviously at cross purposes with the objective of suppressing charge build-up on objects.
Other prior systems for generating air ions at both polarities that are intended for electrostatic charge suppression and/or for air freshening or purification employ much higher cycling rates or indeterminate cycling rates. Prior U.S. Pat. No. 3,711,743, for example, discloses a method and apparatus for controlling electrostatic charge in which a single ionizing electrode is switched between positive and negative high voltage states at a frequency ranging from 60 cycles per second up to 10 KHz. U.S. Pat. No. 2,264,495 teaches switching of a single ionizing electrode between positive and negative ion generation in response to sensors which detect variations in the concentrations of both types of ions in incoming air in order to maintain a predetermined ratio.
Systems of the latter type are effective for controlling air ion concentrations within an adjacent region but the distance or range at which such control is realized is undesirably limited. Ions of opposite polarity are electrostatically attracted and can then neutralize each other by charge exchange. In devices of the above discussed type, outward dispersion of the ions tends to be relatively slow in relation to the cycling time. Intermixing of the alternating groups of positive and negative ions and consequent neutralization begins in the immediate vicinity of the ionizing electrode and progresses rapidly as the ions disperse outwardly. Thus the ion content in the air falls off rapidly at increasing distances from the ionizer. The effect is most pronounced where both types of ion are generated at the same electrode but also occurs to an undesirable extent in prior systems having separate, spaced apart pairs of electrodes each of which generates a single type of ion.
Various techniques have heretofore been used to extend the effective range of such ionizers but such measures are of limited effectiveness or are subject to other problems. It is a common practice, for example, to provide an air flow past the ionizing electrode with a blower or by situating the electrode in a pre-existing air flow in order to accelerate the movement of newly generated ions away from the ionizer. While this is effective up to a point, the rate of air flow must usually be limited to avoid discomfort of personnel and dislocation of light unattached objects in the room. In another approach to the problem, a large number of ionizing electrodes are arrayed on a supporting gridwork which spans the region in which static charges are to be suppressed, each electrode being connected to the alternating high voltage supply through high voltage cables. If it is sufficiently large, such a system can blanket an area with air ions but it is also bulky, costly and cumbersome to install. The ion neutralization problem remains except insofar as it is compensated for by increased ion production at an increased number of different locations.
Prior systems for generating both positive and negative air ions also tend to be inflexible with respect to accommodating to the needs of different rooms, different specific locations in a particular room and changing conditions in a particular room. Electrostatic charges to be neutralized are not necessarily evenly balanced as between positive charges and negative charges. One type or the other may predominate in a particular room and there may be differences in this respect between different specific locations in the room. Further, the amounts and polarities of static charges to be neutralized within a particular room may change as work operations and the types and locations of equipment are changed. Ionizing systems which generate fixed levels of positive and negative ions in equal amounts or at a fixed ratio during successive fixed time periods cannot function in the most effective manner where variations of these kinds are encountered.
The present invention is directed to overcoming one or more of the problems set forth above.